Photosensitive elements which can be used in image-reproduction processes are well-known in the graphics arts industry. Such elements are exposed to actinic radiation through an image-bearing transparency, such as a color separation transparency, and developed to produce an image which is either positive or negative with respect to the transparency used. Positive-working elements produce an image which is a duplicate of the transparency through which they are exposed. Negative-working elements produce an image that is the reverse of the transparency through which they are exposed. After imagewise exposure, the photosensitive elements are developed by washout of soluble image areas, toning tacky image areas with a colorant, peeling apart photoadherent layers, or combinations of these techniques. A series of such images may be combined to form a color proof.
U.S. Pat. No. 4,489,154 issued to Taylor on Dec. 18, 1984, discloses a dry process for preparing a surprint proof using an imagewise exposed, peel-apart photosensitive element comprising a strippable coversheet; a photoadherant layer containing a colorant; a tacky nonphotosensitive elastomeric layer; and a support. After exposure to actinic radiation through a positive image, e.g. a positive separation transparency, and peeling off the coversheet, a positive colored, photoadherent image remains on the supported elastomeric layer. This image is then adhered to a substrate. By repeating this process with different colored photoadherent layers, a multicolored surprint proof can be built up on the substrate. If exposure is through a negative image, e.g. a negative separation transparency, a positive image is obtained on the coversheet.
Coassigned U.S. patent application Ser. No. 07/357,849 filed on May 30, 1989, discloses a multilayer, peel-apart photosensitive element comprising in sequence a strippable coversheet; and oxyethylene homopolymer photorelease layer; a photohardenable layer, optionally containing a colorant; a tacky nonphotosensitive elastomeric layer; and a support. The photorelease layer reverses the adhesive relationship of the coversheet and the photohardenable layer as previously described by Taylor. After exposure to actinic radiation, and peeling off the coversheet, the unexposed areas of the photosensitive layer adhere more strongly to the coversheet than to the elastomeric layer so that the unexposed, unhardened areas are removed with the coversheet and the exposed, hardened areas remain on the elastomeric layer. If the photosensitive layer contains a colorant, exposure through a positive separation transparency produces a positive, colored image directly on the coversheet. A plurality of such coversheets bearing different colored images can be stacked in register to form a multicolored positive overlay proof. The photosensitive elements are preferably colored yellow, magenta, cyan and black. Such precolored elements can also be used to prepare negative-working multicolor surprint proofs without the use of toners or the additional complications of using an image transfer step or the addition of an adhesive layer.
During stacking of the different colored images, the unphotopolymerized polymer of one color is placed in contact with the coversheet for the next color. The photopolymer tends to stick (or block) to the adjacent coversheet and when the coversheets are separated, the photopolymer image can be ruined. Moreover, the image bearing coversheets have poor slip properties which causes difficulties in obtaining proper registration during formation of the multicolor overlay proof.
In accordance with this invention, it has been found that multilayer, peel-apart photosensitive elements having the requisite slip and antiblocking properties can be obtained by coating the backside of the coversheet of said photosensitive element with a polyolefin coating.